<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      // 回调地狱
      setTimeout(() => {
        console.log(1);
        //---
        setTimeout(() => {
          console.log(2);
          //---
          setTimeout(() => {
            console.log(3);
            //---
            setTimeout(() => {
              console.log(4);
            }, Math.random() * 100);
          }, Math.random() * 100);
        }, Math.random() * 100);
      }, Math.random() * 100);
    </script>
  </body>
</html>
